As ur car have 6.5" screen & radio professional, so u physically have a headunit called "L7 Entry" (infotainment platform shared by F20). CIC is only physically installed if u have professional nav.
I think wht's happening here is, as basic BT function is built into the L7 Entry HU, if u have 6NH then there's no combox. If u have nav pro then, as it doesn't have built-in BT, if u have 6NH then it'll come with a combox with some features disabled.
For the combination of L7 Entry + 6NK/NL, the BT hardware is not present in the headunit as a combox is present, hence a different HU p/n is present.

I think the coding side is fairly easy, but the hardware side is somewhat complicated, at least for me as I am not so much a hardware guy. There are a lot of variables depending on what you are starting with (BT, MULF, TCU, NAV, etc.) I leave that stuff to the more technically adept or experts like BimmerTech and Bimmer Retrofit. I know many people have done it though.